Gelation and vitrification

It should be noted that further reactions and crosslinking may occur after gelation (via molecules and clusters of molecules reacting with the infinite-M gel cluster), but these reactions become governed by diffusion (more so than kinetic) considemtions. 

Gelation, vitrification and phase-separation transitions in curing systems are very well described in the work of GilUiam (1986) via the use of time-temperature-transformation diagrams. 

Like other thermoset resins, cyanate esters are amenable to processing by a large variety of conventional techniques and their processing versatility in contrast to 

Effective use of a thermosetting system requires prediction of the cure kinetics of the system [17] to consistently obtain the maximum and also to predict the flow behaviour of the curing resin, in particular to precisely locate when the sol-gel transition occurs. This is because the polymer can be easily shaped or processed only before the gel point, where it can still flow and can be easily formed with stresses applied relaxed to zero thereafter. Accurate knowledge of the gel point would therefore allow estimation of the optimum temperature and time for which the sample should be heated before being allowed to set the mould. The gel point can also be used to determine the activation energy for the cure reaction of the system [18]. 

The different transformations observed during the cure of epoxy with a stoichimetric amount of curing agent are generally recorded in the form of a time-temperature- rans 

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The chemical structure of the epoxy matrix constituent as well as processing are reported to strongly influence 11 -I3> the thermoset network and hence the properties and durability of the crosslinked polymer 11 ,4-16). The cure of a reactive prepolymer involves the transformation of low-molecular-weight reactive substances from liquid to rubber and solid states as a result of the formation of a polymeric network by chemical reaction of some groups in the system. Gelation and vitrification are the two macroscopic phenomena encountered during this process which strongly alter the viscoelastic behavior of the material. 

Two main transformations can take place during the formation of a polymer network gelation and vitrification. Gelation corresponds to the incipient formation of an infinite network, while vitrification involves the transformation from a liquid or rubbery state to a glassy state (Chapter 4). 

Thermosetting polymers represented in Figs 6.4, 6.5, and also 6.6 were cured at T > Tgw In many cases and particularly for high-Tg networks, a precure step is performed at Tj < Tgco. In this case the two transformations, gelation and vitrification, occur and the rheology in the vicinity of the critical gel point could be affected by vitrification. 

Tg,gel = cure temperature at which gelation and vitrification occur... 

Tgo is the glass transition temperature of the uncured reactants. Below this temperature, in principle the system has no reactivity., eiTg is the temperature at which gelation and vitrification coincide. Between Tp, and geiTg the system will vitrify before... 

F. 4. TTT cure diagram temperature of cure vs. time to the isoviscous event, gelation and vitrification, including TBA and gel fraction data , isoviscous (TBA) O, gelation (TBA) A, vitrification (TBA) , gelation (gel fraction). The system studied was a difunctional epoxy resin, Epon 828 (DGEBA, Shell Chemical Co.), cured with a tetrafimctional aliphatic amine, PACM-20. (bis(p-aminocyclohexyl)methane, DuPont]... 

Fig. 9a and b. TBA spectra for a series of isothermal cures showing changes in (a) the relative rigidity and (b) the logarithmic decrement vs. time. Gelation and vitrification are evident in the 80, 125 and 150 °C scans, but only vitrification is observed in the 200 and 250 °C scans. The system studied was a trifunctional epoxy resin, XD7342 [triglycidyl ether of tris(hydroxyphenyl)-methane, Dow Chemical Co.], cured with a tetrafunctional aromatic amine, DDS (diamin iphenyl sulfone, Aldrich Chemical Co.)... 

Fig. 13 is a TTT cure diagram of three systems a neat epoxy resin and the same epoxy modified with two reactive rubbers at the same concentration level. The times to the cloud point, gelation and vitrification are shown for each system. The cloud point is the point of incipient phase separation, as detected by light transmission. The modified system with the longer times to the cloud point and gelation, and the greater depression of Tg, contains the more compatible of the two rubbers. The difference in compatibility could then be used to account for differences in the volume fractions of the phase separated rubber-rich domains and in the mechanical properties of the neat and the two rubber-modified systems. 

Fig. 13. TXT cure diagram temperature of cure vs. the times to phase separation (doud point), gelation and vitrification for a neat and two rubber-modified systems. of the neat system is also included. The systems studied were DER331/TMAB O, gelation , vitrificaticm modified with IS parts rubber per hundred parts epoxy 1) pr eacted carboxyl-terminated butadiene-acrylonitrile (CTBN) copolymer containing 17% acrylonitrile (K-293, Spencer Kellog Co.) A, phase separation , gelation , vitrification, and 2) polytetramethylene oxide terminated with anmiatic amine (ODA2000, Polaroid Corp.) A. phase separation O, gelation O, vitrification. (DER331/TMAB/ K-293 data from Ref. )...

Although different aspects of the isothermal TTT cure diagram have been presented in this review from an experimental point of view, this section will present some recent work that has attempted to model the cure process. Only the gelation and vitrification processes are examined, and the complicating effects of thermal degrada-... 

With values of Ej/Bm and FJF, it is a simple matter to calculate Pyj, at any value of Tg (= Te ), and then determine the time to vitrification from an assumed kinetic rate law. Using first order kinetics, which seemed to fit the extent of conversion vs. time data, the temperature of cure vs. the times to gelation and vitrification are shown in Fig. 15. The model fits the data well at low temperatures but appears to... 

The detailed physical process of an increase in viscosity followed by gelation and vitrification of network polymers that results from the following crosslinking chemistry is discussed in Section 2.4. 

The polymerization reaction results in the formation of an infinite network so the system gels and then with further reaction vitrifies as the glass-transition temperature, Tg, of the network reaches the reaction temperature. The polymerizing system, which had become diffusion-controlled between gelation and vitrification, then ceases chemical reaction until it is heated further to T > Tg. The rates of gelation and vitrification depend both on the reactivity and on the concentration of the difunctional monomer. 

Figure 3.5 shows an isothermal DSC curve for the curing of TGDDM with the aromatic amine DDS (de Bakker et al, 1993b). Also shown is the NIR analysis of the instantaneous rate of reaction of epoxide. The difference arises due to the change in heat capacity on gelation and vitrification that affects the DSC baseline during cure, as discussed below. 

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